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.\"     From: @(#)socket.2	8.1 (Berkeley) 6/4/93
.\" $FreeBSD: sys/socket.2 186769 2009-01-05 12:18:46Z trhodes $
.\"
.Dd January 5, 2009
.Dt SOCKET 2
.Os
.Sh NAME
.Nm socket
.Nd create an endpoint for communication
.Sh LIBRARY
.Lb libc
.Sh SYNOPSIS
.In sys/types.h
.In sys/socket.h
.Ft int
.Fn socket "int domain" "int type" "int protocol"
.Sh DESCRIPTION
The
.Fn socket
system call
creates an endpoint for communication and returns a descriptor.
.Pp
The
.Fa domain
argument specifies a communications domain within which
communication will take place; this selects the protocol family
which should be used.
These families are defined in the include file
.In sys/socket.h .
The currently understood formats are:
.Pp
.Bd -literal -offset indent -compact
PF_LOCAL	Host-internal protocols, formerly called PF_UNIX,
PF_UNIX		Host-internal protocols, deprecated, use PF_LOCAL,
PF_INET		Internet version 4 protocols,
PF_PUP		PUP protocols, like BSP,
PF_APPLETALK	AppleTalk protocols,
PF_ROUTE	Internal Routing protocol,
PF_LINK		Link layer interface,
PF_IPX		Novell Internet Packet eXchange protocol,
PF_RTIP		Help Identify RTIP packets,
PF_PIP		Help Identify PIP packets,
PF_ISDN		Integrated Services Digital Network,
PF_KEY		Internal key-management function,
PF_INET6	Internet version 6 protocols,
PF_NATM		Native ATM access,
PF_ATM		ATM,
PF_NETGRAPH	Netgraph sockets
.Ed
.Pp
The socket has the indicated
.Fa type ,
which specifies the semantics of communication.
Currently
defined types are:
.Pp
.Bd -literal -offset indent -compact
SOCK_STREAM	Stream socket,
SOCK_DGRAM	Datagram socket,
SOCK_RAW	Raw-protocol interface,
SOCK_RDM	Reliably-delivered packet,
SOCK_SEQPACKET	Sequenced packet stream
.Ed
.Pp
A
.Dv SOCK_STREAM
type provides sequenced, reliable,
two-way connection based byte streams.
An out-of-band data transmission mechanism may be supported.
A
.Dv SOCK_DGRAM
socket supports
datagrams (connectionless, unreliable messages of
a fixed (typically small) maximum length).
A
.Dv SOCK_SEQPACKET
socket may provide a sequenced, reliable,
two-way connection-based data transmission path for datagrams
of fixed maximum length; a consumer may be required to read
an entire packet with each read system call.
This facility is protocol specific, and presently unimplemented.
.Dv SOCK_RAW
sockets provide access to internal network protocols and interfaces.
The types
.Dv SOCK_RAW ,
which is available only to the super-user, and
.Dv SOCK_RDM ,
which is planned,
but not yet implemented, are not described here.
.Pp
The
.Fa protocol
argument
specifies a particular protocol to be used with the socket.
Normally only a single protocol exists to support a particular
socket type within a given protocol family.
However, it is possible
that many protocols may exist, in which case a particular protocol
must be specified in this manner.
The protocol number to use is
particular to the
.Dq "communication domain"
in which communication
is to take place; see
.Xr protocols 5 .
.Pp
The
.Fa protocol
argument may be set to zero (0) to request the default
implementation of a socket type for the protocol, if any.
.Pp
Sockets of type
.Dv SOCK_STREAM
are full-duplex byte streams, similar
to pipes.
A stream socket must be in a
.Em connected
state before any data may be sent or received
on it.
A connection to another socket is created with a
.Xr connect 2
system call.
Once connected, data may be transferred using
.Xr read 2
and
.Xr write 2
calls or some variant of the
.Xr send 2
and
.Xr recv 2
functions.
(Some protocol families, such as the Internet family,
support the notion of an
.Dq implied connect ,
which permits data to be sent piggybacked onto a connect operation by
using the
.Xr sendto 2
system call.)
When a session has been completed a
.Xr close 2
may be performed.
Out-of-band data may also be transmitted as described in
.Xr send 2
and received as described in
.Xr recv 2 .
.Pp
The communications protocols used to implement a
.Dv SOCK_STREAM
ensure that data
is not lost or duplicated.
If a piece of data for which the
peer protocol has buffer space cannot be successfully transmitted
within a reasonable length of time, then
the connection is considered broken and calls
will indicate an error with
-1 returns and with
.Er ETIMEDOUT
as the specific code
in the global variable
.Va errno .
The protocols optionally keep sockets
.Dq warm
by forcing transmissions
roughly every minute in the absence of other activity.
An error is then indicated if no response can be
elicited on an otherwise
idle connection for an extended period (e.g.\& 5 minutes).
By default, a
.Dv SIGPIPE
signal is raised if a process sends
on a broken stream, but this behavior may be inhibited via
.Xr setsockopt 2 .
.Pp
.Dv SOCK_SEQPACKET
sockets employ the same system calls
as
.Dv SOCK_STREAM
sockets.
The only difference
is that
.Xr read 2
calls will return only the amount of data requested,
and any remaining in the arriving packet will be discarded.
.Pp
.Dv SOCK_DGRAM
and
.Dv SOCK_RAW
sockets allow sending of datagrams to correspondents
named in
.Xr send 2
calls.
Datagrams are generally received with
.Xr recvfrom 2 ,
which returns the next datagram with its return address.
.Pp
An
.Xr fcntl 2
system call can be used to specify a process group to receive
a
.Dv SIGURG
signal when the out-of-band data arrives.
It may also enable non-blocking I/O
and asynchronous notification of I/O events
via
.Dv SIGIO .
.Pp
The operation of sockets is controlled by socket level
.Em options .
These options are defined in the file
.In sys/socket.h .
The
.Xr setsockopt 2
and
.Xr getsockopt 2
system calls are used to set and get options, respectively.
.Sh RETURN VALUES
A -1 is returned if an error occurs, otherwise the return
value is a descriptor referencing the socket.
.Sh ERRORS
The
.Fn socket
system call fails if:
.Bl -tag -width Er
.It Bq Er EPROTONOSUPPORT
The protocol type or the specified protocol is not supported
within this domain.
.It Bq Er EMFILE
The per-process descriptor table is full.
.It Bq Er ENFILE
The system file table is full.
.It Bq Er EACCES
Permission to create a socket of the specified type and/or protocol
is denied.
.It Bq Er ENOBUFS
Insufficient buffer space is available.
The socket cannot be created until sufficient resources are freed.
.It Bq Er EPERM
User has insufficient privileges to carry out the requested operation.
.El
.Sh SEE ALSO
.Xr accept 2 ,
.Xr bind 2 ,
.Xr connect 2 ,
.Xr getpeername 2 ,
.Xr getsockname 2 ,
.Xr getsockopt 2 ,
.Xr ioctl 2 ,
.Xr listen 2 ,
.Xr read 2 ,
.Xr recv 2 ,
.Xr select 2 ,
.Xr send 2 ,
.Xr shutdown 2 ,
.Xr socketpair 2 ,
.Xr write 2 ,
.Xr getprotoent 3 ,
.Xr netgraph 4 ,
.Xr protocols 5
.Rs
.%T "An Introductory 4.3 BSD Interprocess Communication Tutorial"
.%B PS1
.%N 7
.Re
.Rs
.%T "BSD Interprocess Communication Tutorial"
.%B PS1
.%N 8
.Re
.Sh HISTORY
The
.Fn socket
system call appeared in
.Bx 4.2 .
